The eruption of Okmok volcano continues. Ash and steam plumes ranging in altitude from 10,000 to 35,000 feet have been observed continuously throughout the week, and intermittent light ash fall has occurred on parts of Umnak and Unimak Islands. From July 28 to July 30 the color code was increased to red in response to a vigorous eruptive episode. On July 31 a fishing boat located about 7 miles north of the volcano reported no visibility due to ash fallout.

Seismic activity continues to be characterized by episodes of continuous and pulsating tremor that are strong enough to be seen clearly on networks of neighboring volcanoes. A small field crew of Alaska Volcano Observatory staff are currently in Unalaska to collect ash and pumice samples, make visual observations, and if possible service the seismic and geodetic network.

Although it is not possible to determine how long the eruption will last, it is likely that eruptive conditions similar to those of the past week will continue for days to weeks and possibly longer. Stronger explosive activity could resume at any time with little or no warning. During the eruption AVO is staffed 24 hours per day and can be reached at 907-786-7497. We have begun posting more frequent updates about activity at Okmok on our web page at http://www.avo.alaska.edu/activity/Okmok.php

Okmok volcano is a 6-mile-wide caldera that occupies most of the eastern end of Umnak Island, located 75 miles southwest of Dutch Harbor in the eastern Aleutian Islands. Okmok has had several eruptions in historic time typically consisting of ash emissions occasionally to over 30,000 feet ASL but generally much lower; lava flows crossed the caldera floor in 1945, 1958, and 1986. The last eruption occurred in February 1997 and was characterized by lava flows and intermittent ash emissions.

The nearest settlements are Nikolski, population about 35, roughly 45 miles west of the volcano, and a small number of people at the abandoned Fort Glenn military base 10 miles east of the volcano.

The eruption of Cleveland volcano continues. Satellite images have shown a strong thermal anomaly through clouds throughout the past week. This suggests the presence of a lava flow at the surface. In the past, persistent thermal anomalies at the surface, especially larger ones, were followed by large ash emissions. Although this is not always the case, it is possible that the unrest at Cleveland volcano could proceed to an explosive ash emission.

Cleveland volcano forms the western half of Chuginadak Island, a remote and uninhabited island in the east central Aleutians. It is located about 75 km (45 mi.) west of the community of Nikolski, and 1500 km (940 mi.) southwest of Anchorage. The volcano's most recent significant eruption began in February, 2001 and had 3 explosive events that produced ash clouds as high as 12 km (39,000 ft) above sea level. This eruption also produced a rubbly lava flow and hot avalanche that reached the sea. The most recent minor ash emissions were observed in October 2006.

OTHER ALASKA VOLCANOES

Seismic activity is monitored in real time at 30 volcanoes in Alaska. Satellite images of all Alaskan volcanoes are analyzed daily for evidence of ash plumes and elevated surface temperatures. Some volcanoes may currently display anomalous behavior but are not considered to be at a dangerous level of unrest. Augustine, Iliamna, Redoubt, Wrangell, Gareloi, Great Sitkin, Makushin, Fisher, Shishaldin, Isanotski, Pavlof, Veniaminof, Ugashik-Peulik, Griggs, Snowy, Fourpeaked, Aniakchak, Tanaga, Kanaga, Akutan, Westdahl, Dutton, Ukinrek Maars, Martin, Mageik, Trident, Katmai, Novarupta, Spurr, and Korovin volcanoes are in color code GREEN and volcano alert level Normal. All are at or near normal levels of background seismicity. AVO did not detect ash plumes or significant elevated surface temperatures in the vicinity of any volcano.

The Alaska Volcano Observatory is a cooperative program of the U.S. Geological Survey, the University of Alaska Fairbanks Geophysical Institute, and the Alaska Division of Geological and Geophysical Surveys.

VOLCANO ALERT LEVELS

NORMALVolcano is in typical background, noneruptive state or, after a change from a higher level, volcanic activity has ceased and volcano has returned to noneruptive background state.

ADVISORYVolcano is exhibiting signs of elevated unrest above known background level or, after a change from a higher level, volcanic activity has decreased significantly but continues to be closely monitored for possible renewed increase.

WATCHVolcano is exhibiting heightened or escalating unrest with increased potential of eruption, timeframe uncertain, OR eruption is underway but poses limited hazards.

WARNINGHazardous eruption is imminent, underway, or suspected.

AVIATION COLOR CODES

GREENVolcano is in typical background, noneruptive state or, after a change from a higher level, volcanic activity has ceased and volcano has returned to noneruptive background state.

YELLOWVolcano is exhibiting signs of elevated unrest above known background level or, after a change from a higher level, volcanic activity has decreased significantly but continues to be closely monitored for possible renewed increase.

ORANGEVolcano is exhibiting heightened or escalating unrest with increased potential of eruption, timeframe uncertain, OR eruption is underway with no or minor volcanic-ash emissions [ash-plume height specified, if possible].

REDEruption is imminent with significant emission of volcanic ash into the atmosphere likely OR eruption is underway or suspected with significant emission of volcanic ash into the atmosphere [ash-plume height specified, if possible].